Zinc oxide is widely used in the various industrial fields such as rubber accelerators, pigments for coating compositions and inks, such electronic components as ferrite and varistor, medicinal products and cosmetics. As one of various applications of this zinc oxide, an exoergic filler has been proposed (see Patent Documents 1 to 4).
However, alumina and aluminum nitride are usually used widely as the exoergic filler. Therefore, the zinc oxide is put into actual utilization less than these fillers.
However, alumina has a problem that kneading machines become extremely worn in the production process of exoergic sheets and so on, because Mohs hardness of alumina is high. Further, it is difficult to add aluminum nitride to a resin in high concentration, because of poor filling property. In addition, aluminum nitride is expensive, so exoergic parts made thereof is expensive. Therefore, new exoergic fillers which are made of other materials than such conventional materials are needed.
Zinc oxide has almost intermediate thermal conductivity between alumina and aluminum nitride and is suitable for use as an exoergic filler. However, zinc oxide that is used widely for industrial use is a fine particle having a particle diameter of 1 μm or less and such zinc oxide is infrequently-used because interfacial area of the particle increases, leading to increase in thermal resistance between each particle and degradation in exoergic property. It is not preferred because the fine particle having a particle diameter of 1 μm or less has large surface area and the viscosity of a resin composition comprising the particle increases to make impossible for the fine particle to mix in high concentration. The particle diameter of zinc oxide is preferably larger because it can be expected that heat transfer route in the resin composition is increased as the particle diameter grow larger and improvement in heat transfer derived from closest-packing effect is attained by combination other filler. Further, it is preferred from a point of view of high heat transfer property to have less hole and high density.
Concerning zinc oxide particles having a median size (D50) of 20 to 120 μm, the one disclosed in Patent Document 5 for varistor is publicly known. When the zinc oxide particle is used as an exoergic filler, it is needed for the zinc oxide particle to have a large particle diameter and have high density of particle inside. However, the zinc oxide particle disclosed in Patent Document 5 has the following characteristics; (1) it shows an aggregate-like shape and has many unevenness and fine pores, so uniform particles can not be obtained; (2) the zinc oxide is designed to increase the number of primary crystal grains at the surface of the particle and inside thereof. For making the zinc oxide more suitable as the exoergic filler, it is preferred to clear the unevenness and fine pores at the surface of the particle and decrease the grains at the surface of the particle and inside thereof to be densified. Therefore, the zinc oxide particle disclosed in Patent Document 5 is not intended for the use as the exoergic filler. In addition, it is not preferred to use the zinc oxide particle as a material for an electronic device because the zinc oxide particle contains aluminum and insulation thereof tends to decrease. That is, a carrier carrying a charge of ZnO is a free electron and has the feature of N-type semiconductor. The conductivity would be improved by adding Al3+ into ZnO, because it works as a donor providing a free electron to Zn2+ and the number of the free electrons in ZnO increases.
As a spherical zinc oxide particle, the one that proposed for the field of functional materials such as varistor (Patent Document 5) and self-cleaning powder (Patent Document 6) which show ultraviolet absorption effect, catalyst effect, antibacterial effect or conductive effect is publicly known. However, it was hard to obtain a zinc oxide particle being spherical and high-density and having a median size (D50) of 17 to 10000 μm by using the above technology.
The method for producing a spherical zinc oxide particle having particle diameter of 1 to 50 μm by baking an organozinc compound has been proposed (Patent Document 7). A spherical particle could be obtained by using the technology, but it could not to obtain spherical particles selectively because konpeito-shaped particles were produced at the same time. The inventor of the present disclosure made an additional test and it found that the particle size distribution of the obtained particle was measured and the median size (D50) was several micrometer, so particles having median size (D50) of 10 μm or more could not obtained.
A zinc oxide being a zinc oxide particle doped by a monovalent dopant is disclosed in Patent Document 8. However, only the zinc oxide of which the outer circumference is doped by a metal is disclosed and a zinc oxide particle having specific shape and density is not disclosed.
In Patent Document 9, an oriented zinc oxide-type piezoelectric material containing at least one selected from the element group consisting of Ca, Mg, Ni and Cu in the composition is disclosed. However, a zinc oxide having particle shape is not disclosed in the Document.